Make sure we are running CI on Java 8

[nedtwigg]

I think that we're running CI tests on Java 8 like so

spotless/.circleci/config.yml

Lines 115 to 125 in 2b62d00

	executor:
	name: win/default
	shell: cmd.exe
	steps:
	- checkout
	- run:
	name: install
	command: choco install ojdkbuild8
	- run:
	name: gradlew check
	command: gradlew check --build-cache -PSPOTLESS_EXCLUDE_MAVEN=true

along with the NPM-only tests here

spotless/.circleci/config.yml

Lines 92 to 107 in 2b62d00

	test_npm_8:
	<< : *env_gradle
	environment:
	# java doesn't play nice with containers, it tries to hog the entire machine
	# https://circleci.com/blog/how-to-handle-java-oom-errors/
	# try the experimental JVM option
	_JAVA_OPTIONS: "-XX:+UnlockExperimentalVMOptions -XX:+UseCGroupMemoryLimitForHeap"
	docker:
	- image: cimg/openjdk:8.0-node
	steps:
	- checkout
	- *restore_cache_wrapper
	- *restore_cache_deps
	- run:
	name: gradlew npmTest
	command: ./gradlew npmTest --build-cache

@tbroyer pointed out that if that were the case, this test (ecca2b8) should have been failing for months.

We should track this down someday to confirm what's going on.

[nedtwigg]

Along these same lines, the test results from test_npm_8 aren't getting slurped by CircleCI properly, as discovered in #900. We fix that too.

[nedtwigg]

Also, we should adopt https://github.com/gradle/test-retry-gradle-plugin so we can spend less time on flaky test failures.